1. Field of the Invention
The present invention relates to surfaces used to reflect light, and particularly to highly light reflectant surfaces that provide even diffusion of light for the purpose of maximizing efficiency for recessed compact fluorescent downlights and the like.
2. Description of Related Art
The present invention generally relates to the field of lighting, and more particularly to luminaires which utilize a reflector for redirecting light. The term "luminaire" as used herein is meant to describe a complete lighting unit comprising a lamp or lamps together with components designed to redirect and distribute light from the lamp(s), along with a housing which positions and protects the lamp(s) and components.
Luminaires can be categorized by numerous methods. Typically, there are indoor and outdoor luminaire applications. Indoor luminaires can be categorized by light output emitted above and below the horizontal. The Commission Internationale de l'Eclairage (CIE) has various luminaire classifications. These are direct, semidirect, general diffuse, semi-indirect, and indirect. Direct lighting defines light that is at least 90% below the horizontal. Semidirect lighting is predominantly downward at 60-90% of the light below the horizontal. General diffuse light describes output where the downward and upward components are about equal. Semi-indirect are predominantly upward with 60-90% of the light above the horizontal. Indirect lighting describes systems are those where 90-100% of the light is directed toward the ceiling and upper side walls.
Within these categories, there are many applications. Some typical applications include recessed luminaires such as fluorescent troffers, incandescent and compact fluorescent downlighting, and high intensity discharge (HID) downlighting. There are also ceiling-mounted luminaires, track mounted, wall-mounted, suspended, and portable luminaires.
Outdoor luminaires can be categorized by similar methods. Light distribution of luminaires is categorized by the Illuminating Engineering Society of North America (IESNA) into five light distribution types. These range from narrow symmetrical distribution to wide non-symmetrical patterns. Some typical applications include pole-mounted luminaires, surface-mounted luminaires, bollard luminaires, and floodlight luminaires.
In virtually all of the luminaire applications mentioned above, light is redirected by the use of a reflective material. The reflection characteristics of these materials can be described as being either specular, diffuse, or a combination thereof. While good mirrored surfaces can provide nearly perfect reflectivity of visible light, the light energy exiting these surfaces does so only at an angle equal to the incident angle of light contact. This type of reflection is referred to as specular reflectance. For many applications it is important that light be reflected with an even distribution of light from the surface. This latter property is referred to as diffuse or "lambertian" reflectance.
Throughout the years, reflective materials for luminaires have been studied, optimized, and specified. Specular materials are typically used in luminaires which are designed to preferentially deliver focused light to specific locations. Diffuse materials are more typically used for even dispersion of light having uniform characteristics without the undesirable high and low intensity light areas which are typically generated through the use of specular reflectors. This is desirable for many applications such as work and home locations where even lighting is preferred. Various methods of utilizing these type of diffuse reflectors are discussed, for example, in U.S. Pat. Nos. 5,192,128 and 5,378,965. There are also many designs which use both specular and diffuse materials to take advantage of both types of reflective characteristics, such as in U.S. Pat. Nos. 5,051,878 and 5,075,827.
In other applications a diffuse reflector coupled with a fresnel lens is utilized to create a uniform directed beam. This is discussed in U.S. Pat. No. 4,504,889.
In still other applications, a diffuse reflector is used in conjunction with diffusers, lenses, or louvers to preferentially direct the light emitting from the luminaire to specific areas. A diffuser scatters the light exiting the luminaire. A lens typically incorporates a series of prisms to preferentially light specific areas of its field of view. A louver is an array of open cells, the walls of which form reflectors. Any of these diffusers, lenses, or louvers can be utilized to redirect light in a wide range of applications, such as 2.times.4 foot fluorescent troffers, compact fluorescent downlights, as well as low and high bay HID warehouse luminaires.
Due to the many requirements, the diffuse reflective material choices are relatively few. The most commonly used diffuse reflective materials are diffuse aluminum, white synthetic enamel paint, and white porcelain enamel paint. According to the Illuminating Engineering Society of North America, these three materials exhibit the highest diffuse reflectance through the listed visible wavelengths of 400 nm, 500 nm, and 600 nm. In these wavelengths, diffuse aluminum ranges from 75-84% reflection, white synthetic enamel paint ranges from 48-85% reflection, and white porcelain enamel paint ranges from 56-84% reflection.
Under these criteria, these typical diffuse reflective materials have a visible reflectance maximum of only 85%. At this reflectance level, 15% of the light which impinges upon the reflector is not utilized. Furthermore, in many applications which use diffuse reflectors, there can be additional loss of light stemming from the multiple reflections which are inherently created with diffuse reflective materials. Depending upon the design of the luminaire and the reflector geometry, there can be as many as fifteen or more multiple reflections of a ray of light before it exits the luminaire. At each reflection point, there is a cumulative loss of light associated with the reflection efficiency of the material. Thus an increase of reflectance efficiency of only a few percent can yield an overall luminaire output increase of as much as 10% to 20% to 50% or higher.
It is further evident that the percent reflection of each material listed varies significantly within its own measured visible wavelengths of 400 nm, 500 nm and 600 nm. This variation can introduce an undesirable color shift between the incident and reflected light. Thus the optimum diffuse reflective material for luminaire applications is one that has consistently high reflectance throughout the visible spectrum.
Due to the many different applications that exist for reflectant materials, it is not surprising that there are many different commercially available products with a variety of diffuse reflective properties. Until the present invention, the highest reflectance material known with excellent diffuse reflectivity was that described in U.S. Pat. No. 4,912,720 and sold under the trademark SPECTPALON by Labsphere, Inc., North Sutton, N.H. This material comprises lightly packed granules of polytetrafluoroethylene that has a void volume of about 30 to 50% and is sintered into a relatively hard cohesive block so as to maintain such void volume. Using the techniques taught by U.S. Pat. No. 4,912,720, it is asserted that exceptionally high diffuse visible light reflectance characteristics can be achieved with this material, with reflectance over previously available reflectant material increasing from 97% to better than 99%.
Despite the reported advantages of SPECTRALON material, it is considered quite deficient in many respects. First, this material comprises a relatively hard block of material that must be carefully carved or machined to desired shapes and dimensions. Commercially, this material is only available in a maximum size of 30.5 cm.times.30.5 cm. This severely limits how and where this material can be used and greatly increases the cost of using this material in many applications, especially where large single piece reflectors are desired. Therefore, where a pliable material is desired (such as with reflectors for fluorescent troffers), the SPECTRALON material plainly is inadequate. Furthermore, the additional machining process provides yet another source for contamination that can be detrimental to its reflective properties.
Second, the SPECTRALON material is apparently limited, both structurally and in its ability to reflect light, to a relatively thick minimum depth (i.e., a thickness of greater than 4 mm). Again, this serves to limit where and how this material can be used. Moreover, this limitation tends needlessly to increase both the amount of material required for a given application as well as the weight of the material required for such application.
Third, the SPECTRALON material is apparently relatively expensive to manufacture and purchase. These costs are only increased by the material's difficulty in processing into the final shape from the hard form (i.e., excessive amounts of material may have to be machined away and discarded during production) and its minimum thickness requirements. As a result, the SPECTRALON material is too expensive to be used in many applications that might otherwise benefit from its reflective properties.
Other materials currently used for coating reflective cavities are reflective paints or coatings based on barium sulfate, magnesium oxide, aluminum oxide, titanium oxide, and other white powders. One such example is Kodak White Reflectance coating No. 6080 which is available from Scientific Imaging Systems of Eastman Kodak Co., Rochester, N.Y. This coating is a specially prepared composition of barium sulfate, binder, and solvent. Despite good initial diffuse reflectance, this material maintains its reflectance properties for only a limited period of time (e.g., for only about six months) under normal atmospheric conditions. The material is expected to be stable for even shorter periods of time when exposed to high intensity ultraviolet radiation. Furthermore, application of this coating is extremely laborious requiring 4-8 coats to ensure an adequate thickness for best reflectance. Storage, preparation, and application of the material also requires special care. Even after all of the necessary steps for application, it still does not guarantee uniform results.
Accordingly, there is a distinct need for a highly diffuse reflective surface that can be easily handled and installed and provide other distinct advantages over existing reflective surfaces used in luminaires.
One particular application which can benefit from a high diffuse reflective surface is that of a recessed compact fluorescent downlight. Compact fluorescent lamps are widely used as an energy efficient substitute for conventional incandescent lamps. In order to provide replacements for incandescent flood lights, a compact fluorescent lamp (CFL) with its required ballast is packaged within a housing or cavity having a reflective surface. One application for such replacement lighting device is within a housing recessed within a ceiling. Another application for this replacement lighting is in architectural track lighting. Another use of a CFL is within a recessed fixture which incorporates its own reflector and ballast. This allows for easy replacement of the CFL without replacement of the ballast and reflector.
The problem with such compact fluorescent lamps is that, in contrast to an incandescent lamp which produces light at essentially a point source, the fluorescent lamp is elongated along an axis and emits its light from coated surfaces in a direction perpendicular to its elongated axis. Because of the configuration of recessed lighting fixtures, the elongated axis of the fluorescent lamp tubes is aligned with the direction in which the light rays are to be directed. In other words, the majority of the light emitted from the tube is directed into the recessed reflector, and not directly out of the lamp. Therefore, in order to be useful, as much light as possible must be efficiently reflected by a reflector from an emitted direction, perpendicular to the axis of elongation of the tubes, to a direction aligned with the axis of elongation.
Some designs attempt to overcome this problem by mounting the elongated axis perpendicular to the direction in which the light rays are to be directed. This configuration creates a new problem in which a large percentage of the light is directed away from the luminaire aperture which then must be redirected around the lamp(s).
Replacement floodlights, as well as all other recessed compact fluorescent applications have traditionally employed the same reflective surface materials and design of incandescent lamps. Such design makes use of the focusing characteristics of a parabolic surface. However, since the characteristics of the fluorescent lamp are different from those of incandescent lamps, such parabolic reflectors are inadequate to meet the needs of recessed compact fluorescent downlights. As a CFL light source becomes further recessed in a reflective cavity such that the lamp becomes more substantially surrounded by the reflective cavity, an increased average number of inter-reflections are required for the light to exit. For each inter-reflection (i.e., light bounce) required, there is a loss of light corresponding to the efficiency of the reflective material. For instance, a reflective material having a reflective efficiency of 96% would lose 4% of the light per bounce compared to a material of 98% reflectance, which loses only 2% of the light per bounce. Thus the 96% reflectant material loses twice as much of the light per bounce as the 98% reflectance material. Therefore, as the depth of the reflective cavity increases, further surrounding the lamp, the number of inter-reflections increases. In this situation, the efficiency of the reflective material becomes increasingly critical to maximize the light output of the system.
One such previous attempt to overcome these problems is described in U.S. Pat. No. 5,363,295. This patent describes the use of a specular reflector which has segments that are shaped to individually reflect the light from each of the multiple fluorescent tubes. This design is useful for concentrating the light into a center beam but still suffers from relatively low efficiency of total light output.
A light source which is "substantially surrounded" by a reflector defines a category of reflective cavities that has an aperture to depth ratio of equal to or less than 2.0. The equation used to calculate this ratio is defined as: EQU R.sub.AD =CA.sub.min .div.CD.sub.max
where:
R.sub.AD =aperture to depth ratio PA1 CA.sub.min =minimum dimension of cavity aperture PA1 CD.sub.max =maximum dimension of cavity depth
Designs which employ reflective cavities having R.sub.AD ratios of 2.0 or less can be greatly enhanced by a highly efficient reflective material; with ratios of less than 1.5, and particularly less than 1.0, are even further benefited.
Therefore, for recessed downlighting and track lighting applications which utilize a CFL light source which is substantially surrounded by a reflective cavity there exists a need for highly efficient reflective materials and designs in order to make optimum use of the compact fluorescent lamp as a light source in a recessed cavity.